This invention relates to a servomechanism that uses piezoelectric elements and that may be used in the piston mechanism for controlling the fluid pressure in the wheel cylinder of a vehicle braking apparatus, the driving mechanism for a fluid pressure control valve, or for the drive mechanism of an injection valve, etc.
Piezoelectric elements vary in volume with the application of a voltage and this property has been used in various applications as the driving force in mechanical apparatuses. In particular, since these piezoelectric elements have a low thermal expansion coefficient and generate a large mechanical force, their use has been proposed for the drive mechanism of fuel injection valves in the severe conditions found in automobile engines, as shown in Japanese Patent Disclosure 58-152161.
However, these piezoelectric elements are used in combination with other members such as a valve member for opening and closing the fuel passage in a fuel injection valve so that while the thermal expansion characteristics of the element itself may be good, the characteristics of the other members in the device will have a strong influence on its operation. For example, if there is a large difference in the thermal expansion coefficients of the piezoelectric elements and the other members, there will be a large difference in the amount of displacement making it difficult to produce a displacement in the working member corresponding to the displacement of the piezoelectric element.
The relationship between the voltage applied to the piezoelectric element and the mechanical displacement has a large hysteresis feature and, accordingly, the amount of displacement obtained by the application of a particular voltage value cannot be readily determined, making accurate control of the displacement based on voltage alone difficult. It is, for example, very difficult to precisely control the degree of opening of a valve.
Furthermore, the relationship between the thickness of the elements, which are formed of laminated plates, and the amount of displacement obtained from the application of a given voltage is set, so with small piezoelectric elements a sufficient displacement cannot be obtained even with the application of a large voltage. It is, accordingly, difficult to obtain sufficient mechanical displacement to drive the valve plug in a hydraulic control valve, for example.